A variety of dosing devices for liquid or semi-viscous products are known for use in automated filling machines. These valves serve to allow a predetermined amount of product to flow from a product supply reservoir into a container.
One valve that is commonly used in automated filling machines is a piston valve. A piston valve typically includes an elongated, cylindrical housing with an outlet at the bottom end of the housing. An inlet in the side of the housing above the outlet allows product to flow into the housing. A valve seat defines the separation between the inlet and the outlet. A plug sits against the outlet side of the valve seat. The plug is attached to a rod which runs coaxially with the housing from the plug to a pneumatic rod drive attached at the top of the housing. When the rod is forced downward by the rod drive, the plug disengages the valve seat allowing product to flow from the inlet to the outlet. This downward position is held until the desired amount of product has flowed through the valve. Then the rod drive pulls the rod upwards forcing the plug against the valve seat. This prevents product from flowing through to the outlet until a new container is in place for filling.
The shelf life of liquid products such as milk or juice is directly related to the amount of contaminants allowed to come into contact with the product before being sealed in the sterile environment of a container. The greater the amount of the contaminants, the lower the shelf life. Therefore, maintaining a sealed aseptic environment for the product as it passes through a valve is desirable. It is also desirable to maintain a seal between the liquid product and the remaining mechanical portions of the valve to avoid fouling of the valve or corrosion of its parts.
The manufacturers of piston-type dosing valves have attempted to solve the above mentioned problems by attaching a flexible diaphragm around the rod to the interior wall of the housing above the inlet thereby sealing the mechanical portion of the valve, including the pneumatic rod drive, from the path of the product. The diaphragm is an effective seal as long as the diaphragm remains intact but, because the diaphragm is continuously being stretched by movement of the rod and pressure of the product, the diaphragm material tends to fatigue. This creates the further problem of detecting a breach in the diaphragm seal. Currently, the only way to accomplish such detection is by shutting down the filling machine and disassembling the valve. Machine downtime and labor of a machine technician add to the cost of the final product.